** FILE ** In this March 25, 1993 file photo, Bosnian Serb leader Radovan Karadzic talks to reporters at United Nations headquarters. Former Bosnian Serb leader Radovan Karadzic, accused architect of massacres making him one of the world’s top war crimes fugitives, was arrested on Monday evening July 21, 2008 in a sweep by Serbian security forces, the country’s president and the U.N. tribunal said. (AP Photo/Marty Lederhandler, File)

BELGRADE, Serbia — Former Bosnian Serb President Radovan Karadzic, alleged architect of massacres and the politician considered most responsible for the deadly siege of Sarajevo, was arrested Monday evening in a Serbian police raid, ending his 13 years as the world’s most wanted war-crimes fugitive.

His alleged partner in the persecution and “cleansing” of tens of thousands of Bosnian Muslims and Croats, former Bosnian Serb military chief Ratko Mladic, remained at large.

A psychiatrist turned die-hard Serbian nationalist politician, Karadzic is suspected of being the mastermind of mass killings that the U.N. war-crimes tribunal described as “scenes from hell, written on the darkest pages of human history.” They include the 1995 massacre of 8,000 Muslims in Srebrenica, Europe’s worst slaughter since World War II.

“This is a very important day for the victims who have waited for this arrest for over a decade. It is also an important day for international justice because it clearly demonstrates that nobody is beyond the reach of the law,” said Serge Brammertz, the tribunal’s head prosecutor.

A Serbian police source, speaking on condition of anonymity because he was not allowed to talk to the media, said Karadzic was arrested in a Belgrade suburb after weeks of surveillance of his safe house and a tip from a foreign intelligence service.

Serbian President Boris Tadic’s office said Karadzic has been taken before the investigative judge of Serbia’s war-crimes court, a legal procedure that indicates he could soon be extradited to the U.N. court at The Hague, Netherlands. Investigative judge Milan Dilparic said early today that Karadzic was “being questioned.”

However, it was unclear whether Belgrade planned to extradite him to The Hague for trial by the U.N. tribunal or attempt to try him in Serbia. Many Serbs consider the tribunal to be biased against them, but Serbia would gain international favor by handing Karadzic over to the U.N. court.

If Karadzic is transferred, he would be the 44th Serb suspect extradited to the tribunal. The others include former President Slobodan Milosevic, who was ousted in 2000 and died in 2006 while on trial on war-crimes charges.

Serbia braced for a possible reaction from ultranationalists who are believed to have helped shelter Karad zic and Mladic over the years.

Heavily armed special forces were deployed around the war-crimes court in Belgrade as dozens of Karad zic supporters gathered nearby.

Several were arrested after attacking reporters in front of the courthouse. Police also deployed throughout central Belgrade and in front of the U.S. Embassy, which was targeted in nationalist rioting over Kosovo’s declaration of independence in February.

The White House called the arrest “an important demonstration of the Serbian government’s determination to honor its commitment to cooperate with the International Criminal Tribunal.”

In the Bosnian capital of Sarajevo — a focus of Bosnian Serb attacks during the war — streets were jammed late Monday as Bosnian Muslims celebrated the arrest.

Srebrenica was one of the final chapters of the 1992-95 Bosnian war, which broke out when ethnic Serbs revolted against a government dominated by Muslims and Croats that declared the republic independent from the disintegrating Serb-dominated Yugoslav foundation. Slovenia and Croatia already had broken away, the latter in another bloody ethnic conflict.

Serbia has been under increasing international pressure to find Karadzic and turn him over.

Still, his arrest came as a surprise to many. His whereabouts had been a mystery to U.N. war-crimes prosecutors. Richard Holbrooke, the former U.S. ambassador who negotiated an end to the Bosnian war, told CNN he calculated that Karadzic is responsible, directly or indirectly, for the deaths of 300,000 people, because without him, there would have been no war or genocide.

The charges against him, last amended in May 2000, include genocide, extermination, murder, deportation, inhumane acts and other crimes against Bosnian Muslim, Bosnian Croat and other non-Serb civilians in Bosnia during the war.

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